1. Field of the Invention
The present invention relates in general to a preparing method of silica slurry for use in wafer polishing. More particularly, the present invention relates to a method of preparing silica slurry for wafer polishing by allowing commercially available silica particles as a seed to be grown. This method provides the slurry of silica particles having improvements with respect to a shape irregularity and a size limitation with silica particles prepared by a commonly used method in the prior art, such as a vapor phase method or a precipitation method among others.
2. Description of the Prior Art
A dispersed system of particles having a uniform size and shape is used in very wide applications such as catalysts, fine ceramics, paints, drugs, and the like, and thus is very high in its industrial value-added. A monodisperse system having high phase stability has advantages of uniform optical and magnetic colloidal properties. For these reasons, many studies have been conducted to prepare particles having a uniform size distribution of a few micrometers to a few nanometers.
Moreover, a sol-gel method, which is much studied in an area of inorganic materials, advantageously provides particles that are excellent in chemical uniformity in a multi-component system while having high specific surface area even at relatively low sintering temperatures.
Polymerization of tetraethylorthosilicate (hereinafter, referred to as TEOS) by the sol-gel method varies as an acid or base catalyst being used. Generally, the use of the acid catalyst produces gels of the three-dimensional network structure other than the growth into particles, whereas the use of the base catalyst, commonly ammonia or ammonium hydroxide, provides spherical particles.
Studies to prepare monodisperse spherical particles using the sol-gel method have been performed by many researchers. Stober, et al. have described the preparation of spherical particles of a uniform size by hydrolysis and polycondensation of tetraalkoxysiliane in an alcohol solvent in the presence of a base [See, J. Colloid and Interface Sci. 26, 26-62 (1968)]. However, the spherical silica particles thus prepared are disadvantageous in that they are very low in their weight fraction, and TEOS ((Si(OC2H5)4), a reactant, is very expensive. For this reason, these silica particles are hardly used in industrial applications, and are used only in limited, special applications, such as model particles for a study or experiment.
Since the study by Stober, et al., there have been studied effects of various variables on the Stober""s reaction, such as concentrations of tetraalkoxysilane, water and ammonia, and reaction temperature.
Bogush, et al. have reported a growth of silica particles and an increase in their weight fraction by hydrolysis and polymerization of TEOS using the silica particles prepared by Stober, et al., as a seed [See, J. Non-Cryst. Solids. 104, 95-106 (1988)]. Moreover, they have presented a correlation between a concentration of each of reactants and a particle size. The correlation equation was obtained through the growth reaction of particles using various concentrations of TEOS, ammonia and water.
In addition, Coenen et al. have reported the preparation of a silica particle-dispersed system of more uniform size using colloidal silica particles (commercially available under Ludox AS-40) as a seed [See, J. Colloid and Interface Sci., 124, 104-110 (1988)]. However, the particles prepared by this sol-gel method have the following two problems. First, they are low in their concentration in the dispersed system. Second, they must be dispersed in an aqueous solution again, because of the use of an alcohol solvent.
Meanwhile, a commercially available Ludox prepared by a precipitation method is composed of very small silica particles in which primary particles are about 20 nm in size. Also, the Ludox has a relatively wide size distribution as compared to that of particles prepared by the sol-gel method. Moreover, fumed silica prepared by the vapor method has disadvantages of a primary particle size of less than 40 nm, a wide particle size distribution, and an irregular particle shape.
We have conducted a study to resolve the problem of low production efficiency that is a drawback of the sol-gel method and to prepare monodisperse silica particles of a desired size. As a result, we have found a method capable of preparing a silica slurry for wafer polishing, which method comprises pre-treating commercially available fumed or colloidal silica particles having a size of several tens nm so that these particles can be uniformly grown in a subsequent particle growth step; allowing the seed silica particles to grow by a sol-gel method; and post-treating the grown silica particles so that these grown particles can be used for wafer polishing. Based on this discovery, we have perfected the invention.
It is therefore an object of the present invention to provide a preparing method of a silica slurry, which comprises growing relatively inexpensive and commercially available fumed or colloidal silica particles as a seed by hydrolysis and polycondensation of TEOS, thereby overcoming drawbacks of the Stober""s method.
It is other object of the present invention to provide a preparing method of silica slurry, which comprises using seed particles of a desired size of various sizes while controlling the amount of reactants, thereby allowing grown silica particles of a desired size to be prepared.
It is another object of the present invention to provide silica slurry capable of being used to polish silicon wafers of to various shapes, sizes and surface roughness.
It is still object of the present invention to provide silica slurry capable of being used in a chemical mechanical polishing process for the semiconductor fabrication.
To accomplish the above objects, the present invention provides a method of preparing a silica slurry for wafer polishing, comprising the steps of: combining an aqueous dispersion of silica particles as a seed, TEOS as a precursor, an alcohol solvent, and a base; growing the seed silica particles by hydrolysis and polycondensation of the precursor TEOS; displacing the alcohol solvent containing the grown silica particles dispersed therein with an aqueous solution by a vacuum distillation; and hydrothermally treating the resulting dispersion of the silica particles.